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Temperature Control +/- .5 deg C

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  • Temperature Control +/- .5 deg C

    I am in need of an incubator. Those available on the market do not have PID controllers, or anything fancy, but state that they are able to maintain temperature within 1/2 deg celsius. I have at my disposal, PID controllers, but I believe that setting it to 1/2 deg would result in a constant pulsing, which is hard on the relays controlling the heating element.

    I need the incubator for running hematology tests on wildlife and domestic animals, as well as culturing microbes from infected wounds if resistance is seen in standard antibiotic therapy. All that I would be doing is transferring the culture to an appropriate lab, I don't mess with microbes in concentrated forms...

    I really have no clue how manufacturers are controlling temps to such a fine degree of precision, without the use of fancy electronics. I have plenty of heaters on hand, but really have no experience in controlling temperatures like this, my experience is high temperatures +/- 10 degs.

    Any thoughts?

    An RTD seems like it should work, but so does a simple rheostat, as it's not like the incubator will be setting outside in the elements, it'll be in a breeze free, mostly temperature controlled room.

  • #2
    Dump the relays and use a solid state relay. Problem solved.
    Free software for calculating bolt circles and similar: Click Here

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    • #3
      Yes we did just that for a DoD lab operated by 3M - (I have no idea what they were doing)

      Part of the trick was they had to be constantly circulating the air through a Laminar flow ventalation system. The other part of the trick was a fairly sophisticated control system (PC based) that incorporated "Fuzzy Logic" (which means it Learns) and a whole slew of Algorithm - and yes a PID was one of them

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      • #4
        Precise temperature control is a bit of an art, but it goes something like this.

        Calculate the heat loss at temperature (for discussion, 100 watts).
        Use a heater that's double the loss (thus 200 watt heater).
        You need a relatively large, good thermally conductive plate attached to the heater, to spread the heat. Most of the floor is good, 1/2" Aluminum works.
        The temp sensor must be located VERY close to the heater.
        A fan inside the incubator enclosure is required to eliminate thermal gradients.
        Evan is right about the SSR, and the internal relay in your PID should last 20 years with no load.

        Watlow has some handy info:
        http://www.watlow.com/reference/index.cfm

        Dave J.

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        • #5
          For best results look for a solid state relay that also incorporates zero crossing detection and switching. They are made specifically for heating element control. It eliminates noise generation on the power line as well as radiated RFI and also makes the heating elements last much longer.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            If you have a true PID controller I don't know why this would not be possible.

            It might take a fair amount of tweaking but once you get the P, I, and D variables dialed in it should work fine with out overworking the relay. You might also consider a SSR instead of a mechanical one but keep in mind the failure state of some SSRs is ON!
            "Work hard. Tell everyone everything you know. Close a deal with a handshake. Have fun!"

            -- Harold "Doc" Edgerton

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            • #7
              A good precision controller would know what the controlled temperature is, but also the ambient temperature so it can provide anticipation to induced changes. The problem will be lag in the system, no matter what, so that is also an issue. Since incubators need only control air temperature, the entire loop should deal only with air temperature and ignore surface temperatures.

              Humidity is also critical in incubators, of course. My work in systems like this demonstrated that controlling the atmosphere external to the incubator worked best - the controlled air was then injected into the incubator at low volume. A two-stage system, in other words. I raised 24 turkeys from eggs this way, and don't recall how many chickens, and never had any problems that reveal themselves in the last week (too much humidity produces horrific results inside the egg, frinstance). For all I know that system is still working after all these years (built in 1972).

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              • #8
                Thermal mass

                Another way of compensating for temperature variations is to add some thermal mass such as a water tank with the heating element located inside.
                457863656C73696F7220212000

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                • #9
                  Multiple thermostats

                  Originally posted by RKW
                  You might also consider a SSR instead of a mechanical one but keep in mind the failure state of some SSRs is ON!
                  Which brings up the point that you need multiple thermostats.

                  1) Fancy PID to control the temp

                  2) Cheapo that sets off a mildly annoying alarm when the temp is too low. But not too annoying of an alarm since you'll be listening to it while the system initially heats up.

                  3) Cheapo that sets off a really annoying alarm when the temp is too high and also cuts power to the heating element and/or the whole PID system and/or cuts power to the whole thing.

                  4) A fusible link in series with the heating element in case of total disaster.

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                  • #10
                    Note that traditional PID control assumes linear behavior, bang/bang control of a heating element certainly isn't that....

                    If you really want tight control, pulse-width modulate the heating element w/ that solid state relay Evan suggested. Given the large thermal inertia of the elements, a 10 hz chopper frequency would be more than enough; the duty cycle on the heating element would be a linear function of the temp. error signal, with saturation (either 0% on or 100% on) occurring at a error of perhaps 5 degrees.

                    - Bart
                    Bart Smaalders
                    http://smaalders.net/barts

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                    • #11
                      I got a chance to talk to some of the guys who worked on the first few instances of temperature control for machining awhile back. The following method is still in use at LLNL for their diamond turning machines, and I can only presume other places as well since they set a lot of the early industry standards.

                      Two circuits are used. One is your adjustment line, and in your case it will be heated. However you want to do this is fine. An electric heater would work, with a thermostat. Doesn't have to be particularly accurate. The water in this line circulates constantly to try and keep it homogenous. Doesn't have to be particularly swift, just get it circulating decently.

                      The second line is your control line. Run one end of this line to your radiators/fans, and the other to a series of liquid-liquid heat exchangers. You can make this, none of this stuff is fancy. However, this line must be pumped very quickly; the bigger lines and bigger pump you put on this, the better off you'll be. Don't bother adding tanks or whatever to add mass, that doesn't matter.

                      The idea goes like this. You have a solenoid, on a two or three second pulse width. You monitor your control line's temperature AFTER it has gone through your radiators. Based on this, you set what percentage of the pulse width the solenoid is open for (which may be near 0%). The solenoid, when open, allows the heated fluid to be pumped into the liquid-liquid heat exchangers. The control fluid pumps over these heat exchangers, and drops off (or picks up) whatever corrective energy is necessary to maintain a constant temperature on the radiators in the application area.

                      It takes some fiddling to get the variables right, but once it's set, you should have temperature control far better than .5 deg/c. The main things you'll have to play with are:

                      - the correlation between control line temperature past the radiators, and the application working temp
                      - the valve open time per tenth or hundredth degree difference in desired temp.
                      - The temperature of the heated fluid. The closer this can be to the desired temp, the better and the less oscillation you'll see in the system.

                      There's no fancy logic to this, and it works shockingly well.

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                      • #12
                        Originally posted by toastydeath
                        It takes some fiddling to get the variables right, but once it's set, you should have temperature control far better than .5 deg/c. The main things you'll have to play with are:

                        - the correlation between control line temperature past the radiators, and the application working temp
                        - the valve open time per tenth or hundredth degree difference in desired temp.
                        - The temperature of the heated fluid. The closer this can be to the desired temp, the better and the less oscillation you'll see in the system.

                        There's no fancy logic to this, and it works shockingly well.
                        Fuzzy Logic was able to learn and apply all of your values AND constantly update and fine tune them as well - thats why we used it. The more data aquired the more accurate it became.

                        But as I was trying to illustrate it has now gone way above and beyond the average PID controller. It now requires a Data base and it would really be nice to have more then 1 processor. You'll also want to monitor and maintain data pertaining to the ever changing outside temps as this allows the cooling or heating equipment to ramp up extra storage in anticipation of outside temperature changes

                        Originally posted by toastydeath
                        I got a chance to talk to some of the guys who worked on the first few instances of temperature control for machining awhile back. The following method is still in use at LLNL for their diamond turning machines, and I can only presume other places as well since they set a lot of the early industry standards.
                        and Chuck Mayfield (PhD Mechanical Engineering) learned on the 3M facility (1992)
                        Last edited by JoeFin; 07-07-2009, 06:54 PM.

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                        • #13
                          The setting in the pid controller, called deadband.

                          Hope it helps..

                          I did a binary type arrangement through a triac once.. the diac (exciter) was fed with binary valued incremental resistors.. making it come on in variable stages.. was wonderful melting metal till lightning struck it.. Seems I used 8 resistors and one hex word to control it with a old pc running dos-basic out the parallel port.. Perhaps that was about 88?? 87? I forget..
                          Last edited by Dawai; 07-07-2009, 06:56 PM.
                          Excuse me, I farted.

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                          • #14
                            Originally posted by JoeFin
                            Fuzzy Logic was able to learn and apply all of your values AND constantly update and fine tune them as well - thats why we used it. The more data aquired the more accurate it became.

                            I really wasn't sure if fuzzy logic had stuck around or not. Really have not heard the term from anyone lately. Sounds like PID with AI added in.
                            "Work hard. Tell everyone everything you know. Close a deal with a handshake. Have fun!"

                            -- Harold "Doc" Edgerton

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                            • #15
                              Originally posted by RKW
                              I really wasn't sure if fuzzy logic had stuck around or not. Really have not heard the term from anyone lately. Sounds like PID with AI added in.
                              Perhaps I'm dating myself in regards to Temp control projects with that one. I moved over to petroleum refining since then.

                              Non Sinusoidal Sine Waves (Harmonics) was also a hypothetical argument at the time too. But after the chill water loops shut down when the VFDs over taxed the Ckt Breakers (due to harmonics content) it wasn’t hypothetical to the many PhDs and MEEs working on the project. Damm Chuck was about to throw me under the bus in the board room until I reminded him of the logarithmic nature of what was then a hypothetical argument.

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